1. Field of the Invention
The present invention relates to a physical quantity measuring apparatus that is not affected by temperature changes.
2. Description of Related Art
Some physical quantity measuring systems measure temperatures, stresses, and other physical quantities on the basis of changes in central reflected wavelength in fiber Bragg gratings (hereinafter referred to as FBGs) provided in optical fiber. An arrayed waveguide grating (AWG) is used in these physical quantity measuring systems.
FIG. 11 is a schematic illustration showing a structure of a conventional physical quantity measuring apparatus disclosed in Patent Document 1 (JP-B-3760649). This physical quantity measuring apparatus 201 has a plurality of FBGs 204 in an optical fiber 203 to which measurement light is directed from a wide-band light source 202; the wavelength of light reflected from each FBG 204 is detected so as to measure a physical quantity at a position at which the each FBG 204 is disposed. In the physical quantity measuring apparatus 201 shown in FIG. 11, minute reflected light bands are assigned to the plurality of FBGs 204 in such a way that these bands do not overlap each other, and light reflected from each FBG 204 is directed to an AWG 205 where the light is separated into a plurality of output channels, central wavelengths of which are spaced at very short intervals. A light receiving device (photodiode) 206 is provided for each output channel at the output side of the AWG 205. The wavelength of the reflected light is measured based on the logarithm value of a photo-electric current ratio of two adjacent photodiodes 206. The output channel has light transmission characteristics for a particular wavelength band. The central wavelength in the above light transmission characteristics will be referred to below as the central wavelength of the output channel. The physical quantity measuring apparatus 201 assigns each of the reflected light bands of the plurality of FBGs 204 between the central wavelengths of each two adjacent output, channels of the AWG 205. A divider 207 (ranging from, e.g., DIV 1 to DIV 4) outputs the ratio of the photo-electric currents of two adjacent photodiodes 206 as a log value.
FIG. 13 is a schematic illustration showing the structure of another physical quantity measuring apparatus disclosed in Non-patent Document 1.
Non-patent document 1: S. Kojima, A. Hongo, S. Komatsuzaki, and N. Takeda: “High-speed optical wavelength interrogator using a PLC-type optical filter for fiber Bragg grating sensors”, SPIE's International Symposium on Smart Structure and Materials, Proceedings of SPIE, Vol. 5384, pp. 241-249, 2004.
The physical quantity measuring apparatus 211 shown in FIG. 13 comprises a wide-band light source 212, an optical branching filter 213, an FBG 214, and an FBG wavelength measuring unit 215. The FBG wavelength measuring unit 215 includes an AWG 216 having 40 output channels, photodiodes 217, each of which is provided on the output side of the AWG 216 for each output channel, A/D converters (ADCs) 218 connected to the photodiodes 217 on a one-to-one basis, and a CPU 219 that uses data output from the A/D converters 218 to carry out calculation for physical quantities.
An example of measurement by the use of the physical quantity measuring apparatus 211 shown in FIG. 13 will be described. When changes in central reflected wavelength of the FBGs 214 are measured by the physical quantity measuring apparatus 211, the central reflected wavelength of the FBGs 214 is set up close to the center of the central wavelengths of two adjacent output channels (e.g., output channels A and B) of the AWG 216. In this case, for example, the output channel A has shorter central wavelength than the central reflected wavelength of FBG, and the output channel B has longer central wavelength than that.
FIG. 14 is a graph showing an example of a relationship between a loss and a wavelength of spectrum exhibiting reflection characteristics (reflection spectrum) of the FBG and channel-specific transmission characteristics (transmission spectrum) of the AWG in the prior art. In the figure, it is assumed that the full width of half maximum of the reflection spectrum of the FBG is 0.5 nm, and the distance between the central wavelengths of two adjacent output channels (output channels A and B, for example) of the AWG is 0.8 nm. The reflected light from the FBG 214 is then branched to output channel A and output channel B of the AWG 216, according to its wavelength. If the central reflected wavelength of the FBG 214 is shortened, current, the intensity of which is proportional to the amount of light, that is output from the photodiode 217 corresponding to output channel A of the AWG 216 increases. The voltage output from an A/D converter 218, which converts a current from the photodiode 217 to a voltage and outputs the converted voltage, also increases. Conversely, current, the intensity of which is proportional to the amount of light, that is output from the photodiode 217 corresponding to output channel B of the AWG 216 decreases. The voltage output from the A/D converter 218, which converts the current from the photodiode 217 to a voltage and outputs the converted voltage as described above, also decreases. If the central reflected wavelength of the FBG 214 is prolonged, an operation that will be performed is opposite to the operation performed when a change to a short wavelength occurs.
As described above, the amount of reflected light, which is transmitted to each output channel of the AWG 216, may increase or decrease depending on whether the central reflected wavelength of the FBG 214 is shortened or prolonged. Accordingly, after the transmitted light is converted by the photodiode 217 and A/D converter 218 to a voltage, a change in the central reflected wavelength of the FBG, which is measured in advance, is compared with a ratio between output voltages dependent on the amounts of light form the output channels of the AWG so as to detect a change in the central reflected wavelength of the FBG. The AWG 216 functions as an optical filter that converts a change in the central reflected wavelength of the FBG 214 to an equivalent change in the amount of light. This type of optical filter has no movable part, so it is suitable to a system that need to highly precisely measure changes in the central reflected wavelength of the FBG 214, which occurs at a high frequency.
FIG. 15 is a schematic illustration showing an elastic waveform measuring apparatus. The elastic waveform measuring apparatus 221 shown in FIG. 15 is devised by remodeling the physical quantity measuring apparatus 211 shown in FIG. 13 so that elastic waves can be measured; an FBG 224 and a PZT actuator 223 for generating vibration are provided on, e.g., a carbon fiber reinforced plastic (CFRP) laminated plane 222. Distortion generated in the CFRP laminated plane 222 is detected from a change in the central reflected wavelength of the FBG 224 with respect to the elastic wave from the PZT actuator 223.
FIG. 16 is a time waveform diagram showing an example of an input voltage signal to a PZT actuator. Specifically, when a voltage signal (PZT input) as shown in FIG. 16 is applied to the PZT actuator 223, an elastic wave is transmitted from the PZT actuator 223 to the CFRP laminated plane 222 and then to the FBG 224. Signals are then output from the photodiodes 217 corresponding to output channels A and B of the AWG 216 shown in FIG. 13 to the corresponding ADCs 218, AC components of the signals being represented by output voltage changes. The signals have the same cycle but have opposite polarities, as shown in FIG. 17. FIG. 17 is a time waveform diagram showing output voltages of output channels A and B of the AWG.
When a difference is taken between the outputs from output channels A and B, shown in FIG. 17 (the outputs are converted voltages), a waveform of a differential signal that represents a change in the central reflected wavelength of the FBG 224 as a voltage change is obtained, as shown in FIG. 18. FIG. 18 is a time waveform diagram showing an output voltage of a differential signal between the outputs from output channels A and B, shown in FIG. 17. If the signal includes noise, when measurement is repeated a plurality of times, e.g., 1000 times, and an average is taken to eliminate the noise.
In the channel-specific transmission characteristics shown in FIG. 14, changes in the position of the central reflected wavelength of the FBG 224, which is set up between the central wavelengths of the adjacent output channels of the AWG 216, can be represented as changes in voltage signal amplitude, as shown in FIG. 19. FIG. 19 is a graph showing a relationship between amplitude of signal and a relative distance of FBG in the conventional physical quantity measuring apparatus. FIG. 19 plots, on the horizontal axis, the relative distance from the center of the central wavelengths of two adjacent output channels of the AWG 216 (see FIG. 14) to the central reflected wavelength of the FBG 224. The coordinates of the center of the central wavelengths of the two adjacent output channels of the AWG 216 are set to 0%. The coordinates at which the central wavelength of the output channel having the shorter wavelength of the two adjacent output channels of the AWG 216 matches the central reflected wavelength of the FBG 224 are set to −50%. On the other hand, the coordinates at which the central wavelength of the output channel having the longer wavelength of the two adjacent output channels of the AWG 216 matches the central reflected wavelength of the FBG 224 are set to 50%. The relative distance is obtained by dividing the distance between the central reflected wavelength of the FBG and the central wavelength of output channel A or B, whichever is closer to the central reflected wavelength of the FBG, by the interval between adjacent output channels of the AWG.
In FIG. 19, waveforms that will appear in a zone less than −50% and a zone exceeding 0% can be thought to be nearly symmetric to a waveform in a zone from −50% to 0% when the waveform of an output channel further adjacent to an adjacent output channel of the AWG 216 (for example, an output channel having a short wavelength that is adjacent to output channel A in FIG. 14 or an output channel having a long wavelength that is adjacent to channel B) is considered. Therefore, these waveforms are omitted in FIG. 19.
With the physical quantity measuring apparatus 201 in FIG. 11, the central reflected wavelength of the FBG 204 must be assigned between the central wavelengths of two adjacent output channels of the AWG 205 as mentioned before, and must be also assigned on a straight part (part where nearly liner approximation is possible) in characteristics of the central reflected wavelength of the FBG 204 versus the log value of a photo-electric current ratio between output channels, as shown in FIG. 12. FIG. 12 is a graph showing a relationship between the logarithm value of a photo-electric current ratio and a central reflected wavelength of an FGB. Accordingly, problems described below arise.
If the central reflected wavelength of the FBG 204 changes by more than the interval (separate wavelength bandwidth) between central wavelengths of two adjacent output channels of the AWG 205, it is hard for the physical quantity measuring apparatus 201 in FIG. 11 to detect such a change. This is because if the AWG 205 has output channels with a large width so as to detect a large change in the central reflected wavelength of the FBG 204, the log value changes insensitively with respect to the wavelength, so it is hard to detect a minute change in the central reflected wavelength of the FBG 204. For example, AWGs 205 with output channel widths of 0.2, 0.4, 0.6, 0.8, and 1.6 nm are commercially available at present (see Non-patent Document 2). By contrast, distortion sensitivity and temperature sensitivity of the FBG 204 are respectively about 1.2 pm/microstrain and 10 pm/° C.; when strain of 1400 microstrain or more is measured, the central reflected wavelength of the FBG 204 changes by 1.6 nm or more. Therefore, it is hard to use the above commercial AWGs 205 to measure strain that appears as changes in the central reflected wavelength of the FBG 204.
Non-patent document 2: Homepage of NTT Electronics Corporation, http://www.nel-world.com/products/photonics/awg_mul_d.html (uploaded on Nov. 3, 2006).
Another problem is that, to assign a linear part in the characteristics of the central reflected wavelength of the FBG 204 versus the log value within the central reflected wavelength range of the FBG 204, the central reflected wavelengths of the AWG 205 and FBG 204 must be strictly designed. If the central reflected wavelength of the FBG 204 is likely to change beyond the wavelength range of the AWG 205 due to an effect by temperature or strain, the strain or temperature of the AWG 205 must be adjusted (modulated) so that the central reflected wavelength of the FBG 204 falls within the linear part in the characteristics of the central reflected wavelength of the FBG 204 versus the log value, involving extra work in manufacturing and measuring.
The central wavelength of each channel of the AWG 205 changes by about 10 pm/° C. depending on the temperature. To highly precisely detect the amount of which the central reflected wavelength of the FBG 204 changes, therefore, the temperature of the AWG 205 must be kept fixed, so a heater, Peltier element, or other equipment for keeping the temperature of the AWG 205 fixed must be added. Although athermal AWGs with a central wavelength change of as small as several tens of picometers within a temperature range of 0 to 60° C. are also available (see Non-patent Document 3), the use of an athermal AWG of this type increases the cost to manufacture of the physical quantity measuring apparatus.
Non-patent document 3: Homepage of NTT Electronics Corporation http://www.nel-world.com/products/photonics/ather_awg.html (uploaded on Mar. 13, 2007).
On the other hand, when the physical quantity measuring apparatus 211 shown in FIG. 13 is used in a measuring apparatus as shown in FIG. 15, if the central reflected wavelength of the FBG 224 is located at the center of the central wavelengths of two adjacent output channels of the AWG 216 (the relative distance is 0%) as shown in FIG. 19, the amplitude of the voltage signal is maximized. If the central wavelength of the AWG 216 and the central reflected wavelength of the FBG 224 overlap each other (the relative distance is −50%), the amplitude of the voltage signal is minimized; the amplitude of the voltage signal is just about one-twentieth the amplitude obtained when the relative distance is 0%.
The central reflected wavelength of the FBG 224 changes according to the change of the temperature or strain at the strain measurement place (place where the FBG is disposed). When the central reflected wavelength of the FBG changes, the amplitude of the voltage signal decreases. Accordingly, to prevent the amplitude of the voltage signal from decreasing, the relative distance between the center of the wavelengths of the two adjacent output channels of the AWG 216 and the central reflected wavelength of the FBG 224 must be controlled so that superior sensitivity is obtained in measurement of a change in the central reflected wavelength of the FBG 224. For this purpose, the temperature of the AWG 216 has been preferably adjusted so that the central reflected wavelength of the FBG 224 is located at the center of the central wavelengths of the two adjacent output channels of the AWG 216 during physical quantity measurement.
However, it is hard to always adjust the temperature of the AWG 216. Accordingly, in the physical quantity measuring apparatus as shown in FIG. 13, a large signal amplitude should be obtained without the temperature of the AWG having to be adjusted.